The Human Heart Explained

The heart is a hollow organ made mostly of muscle. Its job is to pump blood through a network of blood vessels to every part of the body, without stopping, for your whole life.

It sits in the middle of your chest, between the two lungs and behind the breastbone, tilted slightly to the left. Its rounded top, where the big blood vessels join it, is called the base. Its narrow lower tip, the apex, points down and to the left. That tilt is why most people feel their heartbeat a little to the left of centre.

The heart is really two pumps joined together. A wall of muscle called the septum divides it into a right side and a left side, so the blood in the two halves never mixes.

• The right side receives oxygen-poor blood from the body and pumps it to the lungs, where it picks up oxygen.
• The left side receives oxygen-rich blood from the lungs and pumps it to the rest of the body.

Both sides beat at the same moment, so every heartbeat drives both journeys at once. The step-by-step route is the subject of the next part of this series.

Each side has two hollow chambers:

• An atrium (plural atria) at the top. The atria are receiving chambers: blood flows into them from the veins.
• A ventricle at the bottom. The ventricles are the main pumping chambers: they squeeze blood out into the arteries.

The two ventricles push out the same amount of blood with each beat, but the left ventricle has a much thicker muscle wall. It has to build up far more pressure, because it pushes blood around the whole body, while the right ventricle only has to reach the nearby lungs.

Valves are flaps of tough tissue that let blood through in one direction only. Each one opens when the pressure behind it is higher and snaps shut if blood starts to flow backwards.

Valve	Where it is
Tricuspid	between the right atrium and right ventricle
Pulmonary	between the right ventricle and the pulmonary artery
Mitral (bicuspid)	between the left atrium and left ventricle
Aortic	between the left ventricle and the aorta

The tricuspid and mitral valves are held in place by strong cords called chordae tendineae, which stop the flaps from being pushed back up into the atria when the ventricles squeeze.

Four kinds of large blood vessel connect the heart to the body and lungs:

• Vena cava: the superior (upper) and inferior (lower) vena cava bring oxygen-poor blood from the body into the right atrium.
• Pulmonary artery: carries oxygen-poor blood from the right ventricle to the lungs.
• Pulmonary veins: four veins that bring oxygen-rich blood from the lungs into the left atrium.
• Aorta: the body's main artery, which carries oxygen-rich blood from the left ventricle to the rest of the body.

A simple rule helps: arteries carry blood away from the heart and veins bring it back.

The heart wall has three layers: a thin inner lining (endocardium), a thick middle layer of heart muscle (myocardium) that does the squeezing, and an outer protective sac (pericardium) with a little fluid in it to reduce friction as the heart moves.

Heart muscle works non-stop, so it needs its own supply of oxygen. The blood inside the chambers does not feed it. Instead, the coronary arteries branch off the aorta and spread over the outside of the heart, delivering oxygen-rich blood to the muscle itself.

Heart diagrams are drawn as if you are facing the person, so the heart's right side appears on your left. Always work out which side is which before you label anything.

Diagrams also colour oxygen-poor blood blue and oxygen-rich blood red. This is only a convention to tell the two apart: real blood is always red, darker when it carries less oxygen.